1. Field of the Invention
Suspended ceilings having a metal grid framework that supports acoustical panels within rectangular enclosures formed by the grid are used extensively in commercial and industrial buildings.
In the event of a fire in an area covered by such a ceiling, it is of great benefit to keep such a ceiling relatively intact, so that the ceiling can act as a fire barrier to the supporting structure above the ceiling.
This invention relates to connectors in a grid for such ceiling that allow the beams in the grid to expand during a possible fire, in a controlled way, so that the ceiling stays relatively intact.
2. Prior Art
Suspended ceilings having metal main and cross beams interconnected into a grid that supports panels are well known. U.S. Pat. Nos. 5,839,246 and 6,178,712, for instance, incorporated herein by reference, show such ceilings.
The grid in such ceilings has, at each grid intersection, a pair of opposing cross beams and a main beam that form a connection. The connection is formed with connectors, generally in the form of clips, on the end of the cross beams that connect through, and with, a slot in the main beam.
Such a connection is shown in co-pending application Ser. No. 10/754,323 for STAB-IN CONNECTOR, filed Jan. 9, 2004, incorporated herein by reference.
Each cross beam in such a connection has a connector at its end that is thrust, or stabbed-in, from opposing sides of the main beam, through the slot in the main beam. The connectors are all identical.
The grid members of such a ceiling are subjected to high heat during a possible fire, creating expansion forces in the beam. If such expansion forces are not relieved in a controlled way, the beam distorts by buckling and twisting, and no longer supports the panel. The ceiling panel drops through the grid openings of the buckled grid, and the effectiveness of the suspended ceiling as a fire barrier is destroyed. The fire then attacks the building support structure. To avoid such a condition, the prior art has sought to relieve the expansion forces in a way that keeps the panels supported during a fire.
The main beams of a grid are generally kept relatively intact during a fire by providing cut-outs along the beam that permit the beam, in a controlled way, to collapse in-line, longitudinally, from the forces of compression created by the fire. Such an arrangement is disclosed, for instance, in U.S. Pat. No. 4,606,166.
In the cross beams, cut-outs are generally not used, since they weaken the beam unduly in these relatively short beams. Also, the relatively large controlled collapse is not necessary in the relatively short cross beams, since the expansion created by a fire is about 1/10 of an inch per foot, so that in a five foot cross beam, which is the maximum length generally used, approximately ½ inch relief is necessary.
One method of relieving the stress forces from a fire in cross beams is to design a connector on the end of the beam that pierces through the main beam, as seen in U.S. Pat. No. 5,839,246, incorporated herein by reference. The beam is thus permitted to expand, relieving forces due to the elevated temperature, and avoiding buckling of the beam.
Another method of relieving stress in a cross beam due to a fire, is to simply let the connectors at the end of the beam bend sidewise, at a bend line, so that, if they bend in the right direction, the beam moves diagonally, permitting the beam to expand in the diagonal direction before it abuts, at each end, the main beams. The length of the bend determines how much the beam can move in the desired direction, and how much the beam can expand. Such a result is shown, for instance, in FIGS. 4, 6, 7, and 13 of this application, the Figures being designated as prior art.
Generally such bend occurs in the prior art at the location where the connector is riveted to the beam through holes formed in the connector, one above another. Such a bend is shown in FIG. 21B of the '246 patent. These holes create a weak point in the connector, and the bend occurs at this weak point, at the end of the beam, causing the end of the beam to move along the main beam. The end of the connector remains in the slot. Such a bend often causes the cross beam to move too much from its original grid position, resulting in a beam displacement so large that it allows the panel to drop out. This is particularly true when the connectors bend in a direction at each end of the beam that translates the beam to a position parallel to its initial position, rather than diagonally wherein the beam still has a tendency to continue to support the panel.
A further problem in the prior art practice of simply allowing the connector to bend at the rivet holes which forms the weakest point in the connector, due to compression forces built up by a fire, is that there is no way of having the bend occur at a predetermined compression force. In some instances, the bend may occur at a force much less than at the optimum compression force of 100 pounds at which optimum relief occurs. At other times, the bend does not occur until a force of two or three hundred pounds is attained, by which time buckling of the cross beam begins to take place.